Popliteal Cyst

= BAKER CYST

[William Morrant Baker (1839–1896), surgeon and governor at St. Bartholomew's Hospital, London]

= synovial cyst in the posterior aspect of knee joint communicating with posterior joint capsule

Prevalence: 19% in general orthopedic patients; 61% in patients with rheumatoid arthritis

Pathophysiology:

formed by escape of synovial effusion into one of the bursae; fluid trapped by one-way valvular mechanism

1. Bunsen-type valve = expanding cyst compresses the communicating channel
2. ball-type valve = ball composed of fibrin + cellular debris plugs the communication channel

Etiology:

1. Arthritis: degenerative, rheumatoid, pyogenic
2. Internal derangement: meniscal / anterior cruciate ligament tears
3. Pigmented villonodular synovitis

• pseudothrombophlebitis syndrome (= pain + swelling in calf)
• cellulitis (after leakage / rupture)

Location:

1. gastrocnemius-semimembranosus bursa = posterior to gastrocnemius muscle at level of medial condyle
2. supralateral bursa = between lateral head of gastrocnemius muscle + distal end of biceps muscle superior to lateral condyle (uncommon)
3. popliteal bursa = beneath lateral meniscus + anterior to popliteal muscle (uncommon)

• communication with bursa (documented on arthrogram)
• well-outlined hypointense collection on T1WI + hyperintense on T2WI
• septa in 50%

Types:

1. Intact cyst
   • smooth contour
2. Dissected cyst
   • smooth contour extending along fascial planes (usually between gastrocnemius + soleus)
3. Ruptured cyst
   • leakage into calf tissues

DDx of other synovial cysts about the knee:

1. Meniscal cyst (at lateral / medial side of joint line; associated with horizontal cleavage tears)
2. Tibiofibular cyst (at proximal tibiofibular joint, which communicates with knee joint in 10%)
3. Cruciate cyst (surrounding anterior / posterior cruciate ligaments following ligamentous injury)